The proportion of 11-year-olds in England who leave primary school unable to write a proper sentence has risen for the first time in 15 years, ministers admitted today.

A fifth of 11-year-olds did not reach the target level in English tests this May, figures from the Department for Children, Schools and Families show.

This represents a one percentage point drop on last year's English results when 81% of 11-year-olds reached the standard expected of their age group (level 4). It is the first time there has been a drop in the results since the Sats were introduced in 1995.

Today's figures also showed that almost two fifths of 11-year-olds are still failing to grasp the finer points of the three Rs. Of the 600,000 pupils who took the national curriculum tests, just 61% reached level 4 in reading, writing and maths – down 1% on last year.

Almost three in 10 (28%) failed to reach this standard in English and maths combined. This was also down 1% on last year and means more than a quarter of 11-year-olds left primary school unable to read and write properly and do basic arithmetic.

While the English results fell, the percentages for those reaching the required standard in maths and science were unchanged at 79% and 88% respectively.

Opposition ministers said the results showed progress in primary schools had stalled and, in the case of English, slipped backwards.

The tests have attracted criticism since their inception. They are used to judge how far a pupil has improved in English, maths and science since they started school. Teachers today described the tests as "meaningless nonsense" and renewed calls to boycott them next year.

Heads and teachers argue that they restrict children's learning. Teachers say they may refuse to prepare pupils for the tests when school starts in the autumn.

The government's target is for 78% of 11-year-olds to reach the standard expected of their age group in English and Maths by 2011. This year 72% attained that benchmark.

Diana Johnson, the schools minister, admitted that reaching the target would be "challenging" and acknowledged that parents and teachers would be concerned that standards in English were slipping.

The results also show that boys are falling far behind girls in writing, and are also lagging in reading and science. Four out of 10 boys could not write a complete sentence using commas, while three-quarters of girls managed this task.

In science, 89% of girls reached the standard expected of them, while 88% of boys did.

But in maths, the situation was reversed, with 78% of girls able to add, subtract, multiply and divide in their heads compared with 79% of boys.

The proportion of 11-year-olds able to write well-ordered English in paragraphs – a task ahead of their age – fell by 1% point this year to just under three in 10 children.

In maths, the proportion who could do simple equations – also viewed as a step further than the level expected of this age group – rose to 35% from 31% last year.

Johnson said the majority of the 11-year-olds who left primary school unable to read, write or do basic arithmetic had special needs or were not native English speakers.

Many of the children who fell behind tended to be in urban and deprived areas, but come from a wide range of schools, the DCSF said.

Johnson said: "Parents are going to be concerned about this blip, as are headteachers. The government is concerned as well. We have plans to deal with this. Clearly there is a need to drill down on what has caused this blip. It is going to be challenging to reach the government's target by 2011."

The government has already started to remedy the problem with more one-to-one tuition for children who are falling behind, and reading and writing programmes targeted especially at boys, she said.

"The main concern is boys' writing being so far behind," Johnson said. "That's where we really need the support and intervention, to get boys writing.

"This year's results demonstrate loud and clear that we are going to have to ask some hard questions and re-double our efforts if we are to make further progress in national curriculum tests next year and in future years."

But teaching unions renewed calls for the tests to be scrapped. The National Union of Teachers (NUT) and the National Association of Head Teachers (NAHT) will ballot their members in the autumn on boycotting next May's tests.

In May, a government-commissioned review into testing recommended that science tests for 11-year-olds be abolished and that teachers assess pupils on the subject instead. This left the door for all tests to be scrapped in future.

The NUT has warned that up to one in five of the test results may be inaccurate.

Mick Brookes, general secretary of the NAHT, said teachers had accused the marking of being erratic. In some cases, pupils were penalised for not dotting their "i"s. Brookes said he would ask the exams regulator, Ofqual, to investigate.

"We believe that the system is simply being maintained through political obduracy and that there are better ways to gain a broad picture of primary education in England," he said.

Christine Blower, the general secretary of the NUT, said: "The national curriculum testing arrangements are long past their sell-by date. They have been proved to be detrimental to children's learning and narrow the curriculum. Teachers are no longer prepared to allow the damage they cause to continue."